Information contained in this section is provided in more detail in U.S. Pat. No. 5,361,710. A brief overview of this technology follows in the section below.
1. Field of the Invention
The present invention relates to incinerators and to a method and apparatus for the active control of compact waste incinerators. More particularly, the invention described herein is an improved technique for the modulation of waste in an actively controlled compact waste incinerator afterburner.
2. Description of the Prior Art
The interaction between turbulent mixing processes and combustion is important in many practical applications such as air-breathing propulsion systems, energy conversion power plants, hazardous waste incinerators and other chemical reactors and industrial processes. Studies of turbulent mixing during the last two decades established the role of organized coherent large-scale vortical structures in the mass and momentum transfer across the shear layer between two fluids in motion. (Please refer to the following papers for a more detailed discussion on this topic: S. C. Crow and F. H. Champagne, "Orderly Structure in Jet Turbulence", J. Fluid Mech., 48, pp. 545-591, 1971. G. L. Brown, and A. Roshko, "On Density Effects and Large Structure in Turbulent Mixing Layers", J. Fluid Mech., 64, pp. 775-816, 1974. C. M. Ho, and P. Huerre, Ann. Rev. Fluid Mech., 16, pp. 365-424, 1984.).
It was further determined that by manipulating these structures it is possible to alter the mixing process. Active control methods were devised to enhance the spreading rate of the shear layer by mechanical or acoustic excitation of the initial shear layer, and thus accelerate the mixing between the two streams. (Please refer to the following papers for a more detailed discussion on this topic: D. Oster, and I. J. Wygnanski, "The Forced Mixing Layer Between Parallel Streams", Fluid Mech., 123, pp. 91-130, 1982. C. M. Ho, and L. S. Huang, "Subharmonics and Vortex Merging in Mixing Layers", J. Fluid Mech., 119, pp. 443-473, 1982.)
The understanding of the mechanism governing turbulent mixing and their control was extended to turbulent combustion. New laser based diagnostic techniques (R. K. Hanson, "Combustion Diagnostics: Planar Imaging Techniques", 21st Int. Symp. on Combustion, pp. 1677-1691. Pittsburgh: The Combustion Institute, 1986.) with high temporal and spatial resolution, which yield species-specific two and three dimensional maps of the combustion region, accelerated the process; the important role of controlling the large and small scale mixing on the combustion process was determined. (E. J. Gutmark, T. P. Parr, D. M. Hanson-Parr, and K. C. Schadow, "On the Role of Large and Small-Scale Structures in Combustion Control", Combustion Science and Technology, 66, pp. 107-126, 1989.). Initial studies of combustion control focused on the problem of combustion instabilities. The numerous studies on the application of active control to suppress combustion instabilities were reviewed recently by Candel (S. Candel, 24th Int. Symp. On Combustion. Pittsburgh: The Combustion Institute, 1992.) and McManus et al. (K. R. McManus, T. Poinsot, and S. Candel, "A Review of Active Control of Combustion Instabilities", Progress in Energy and Combustion Science, 19, pp. 1-29, 1993.). Active control by shear layer excitation was applied to enhance energy release (Please refer to the following papers for a more detailed explanation on this topic: K. R. McManus, U. Vandsburger, and C. T. Bowman, "Combustor Performance Enhancement through Direct Shear Layer Excitation", Combustion and Flame, 82, pp. 75-92, 1990. K. Yu, A. Trouve, and S. Candel, "Combustion Enhancement of a Premixed Flame by Acoustic Forcing with Emphasis on the Role of Large Scale Vortical Structures", AIAA Paper No. 91-0367, 1991. E. J. Gutmark, K. J. Wilson, K. C. Schadow, B. E. Parker, R. L. Barron, G. C. and Smith, "Dump Combustor Control Using Polynomial Neural Networks (PNN)", 31.sup.st AIAA Aerospace Sciences Meeting, Reno, Nev., AIAA Paper No. 93-0117, 1993. K. T. Padmanabhan, C. T. Bowman, J. D. Powell, "An Adaptive Optimal Combustion Control Strategy", 25th Int. Symp. on Combustion, Pittsburgh: The Combustion Institute, 1994.) and to mitigate the production of pollutants (Chen, V. G. McDonell, and G. S. Samuelson, WSS/CI Fall Meeting, Paper No. 92-75, 1992.). Fluid dynamic control has also been applied to hazardous waste incineration (O. I. Smith et. al., "Incineration of Surrogate Wastes in a Low Speed Dump Combustor", Comb. Sci. and Tech. 74, 199 (1990). R. Marchant et. al. "Development of a Two-Dimensional Dump Combustor for the Incineration of Hazardous Wastes", Comb. Sci. and Tech. 82, 1 (1992).).
Incinerator technology relies on a number of factors including combustion temperatures, fuel and waste residence time and the fine scale mixing of the fuel, waste and an oxidizer. Incineration technology includes rotary kilns, fixed and multiple hearth incineration devices, fluidized bed incinerators and liquid injection incinerators. These devices are usually large-scale devices which rely on high heat capacity and long residence time to achieve the required destruction capacity which most incinerators need to operate. These types of devices are able to achieve high residence times and heat capacities by utilizing a very large combustion chamber. Long residence times and high heat capacities result in high operational costs as well as unacceptable emissions when the incinerator is forced to operate outside its optimum design conditions.
Compact incinerators can achieve the required destruction capacity in a small-scale device, with much shorter incineration time and higher combustion efficiency. Compact incinerators are valuable because they eliminate the need to transport waste from remote locations to a central facility and because they can be installed on ships at a minimum of space penalty.
In order to maintain reliable and efficient operation compact incinerators require a highly optimized and effectively controlled combustion process. Achieving increased thermal destruction efficiency while maintaining a minimal amount of particulate and gaseous pollutant emissions is the ultimate goal in designing a compact incinerator unit. There exists a continuing need for a reliable and inexpensive method of controlling the combustion process in order to improve the overall performance of the combustor. Optimally, a compact incinerator will have a very high efficiency and very low emissions.
The combustion characteristics of an enclosed combustor are closely related to the interaction between shear flow dynamics, of the fuel and air flow at the inlet and acoustic modes of the combustor. The airflow dynamics may lead to highly unstable combustion. Unstable combustion may occur when the acoustic modes of the combustor match the instability modes of the incoming airflow. The shedding of the airflow vortices upstream of the combustion chamber tends to excite acoustic resonances in the combustion chamber, which subsequently causes the shedding of more coherent energetic vortices. The presence of such vortices provides a substantial contribution to the overall efficiency of the combustion process.
Historically, passive combustion control was the norm. For example, in the dump combustor, nonstandard inlet duct cross-sections were used to control the generation and breakdown of large-scale vortices. These vortices play a critical role in driving pressure oscillations and determining flammability limits. Passive control has also been achieved by utilizing bluff-body flame holders at the downstream facing step into a dump combustor.
Recently, active combustion control has received more and more attention. In active combustor control various control devices such as actuators are used to modify the pressure field in the system and to regulate the air or fuel supply. Active control devices which have been used in laboratory experiments include loudspeakers to modify the pressure field of the system or to obtain fuel flow regulation, pulsed gas jets aligned across a rearward facing step, adjustable inlets for time-variant change of the inlet area of the combustor and solenoid-type fuel injectors for controlled addition of secondary fuel into the main combustion zone. These active control devices have been successful in suppressing pressure oscillations and extending flammability limits when the combustor operates at low heat release rates. The existing trend in active control techniques for a combustor is towards decreasing performance of the controller with increasing energy levels within the combustor.